Electrolytic treatment of ferrous metal surfaces



United States Patent 3,032,487 ELECTROLYTIC TREATMENT OF FERROUS METAL SURFACES Shigern Yonezaki and Minoru Kamata, Yawata City, Japan, assignors to Yawata Iron and Steel Co., Ltd.,

Tokyo, Japan, a corporation of Japan No Drawing. Filed May 26, 1959, Ser. No. 815,783 Claims priority, application Japan May 30, 1958 6 Claims. (Cl. 204-56) The present invention relates to the surface treatment of ferrous metal products, more particularly, of cold rolled ferrous metal products, such as, sheet and strip, with a view to improving corrosion resistance and paint adherence thereof.

Of known metal treatment processes of prior art, the application of chromic acid is well known to industry. According to the teaching of US. Patent Nos. 2,768,103-4, the formation of a thin film consisting of chromium chromate on the ferrous metal sheet is obtained by the process which comprises subjecting a part of chromate on the surface thereof to the reducing action by means of a reducing agent. And US. Patent No. 2,780,592 teaches that a ferrous metal article is cathodically electrolyzed in the solution containing 100-400 g./l. of chromic acid added with more than 8 g./l. of boric acid ions at room temperature, but a period of more than 20 seconds is required to complete the treatment.

It is also known that the ferrous metal is cathodically electrolyzed in the solution containing chromic acid added with l.2-4.8% phosphate ions having a pH value of from 1 to 2, which, however, results in the formation of chalky films providing inadequate corrosion resistance depending upon the conditions of the treating process, particularly at lower bath temperatures.

In addition, the use of chromic acid on zinc products, such as, zinc-plated steel sheet, to improve its corrosion resistance is also well known to industry. As an example, a well known commercial process includes immersing the zinc-plated metal article in the chromate solution added with such an agent, for example, sulphuric acid or nitric acid, as inhibits to passivate zinc in the chromate solution, in order to reduce the chromate to trivalent chromium ions by the dissolution of zinc in the solution and increase the value of pH at the same time, and producing a thin gel-like film consisting of chromium chromate on the surface of zinc-plated metal article.

However, in the case of ferrous metal articles, the addition of an agent to the electroyltic solution will not inhigit to passivate steel. Furthermore, as iron is dis-. solved a very little in the solution, the same chromate treatment as can be applied to zinc-coated ferrous metal articles has never been carried out to advantage.

In accordance with an embodiment of the present inven tion, a ferrous metal article is cathodically electrolyzed in the chromate solution containing trivalent chromium ions added with boric acid and phosphoric acid, the concentration of which is much lower than that specified in the foregoing patents, and dried. The treatment of our invention may be performed at room temperature since no chalky film is formed at a low temperature. Further, this treating process can impart corrosion resistance as well as lacquer adherence to ferrous metal articles in a considerably shorter time of period. In addition, the ferrous metal article treated with the process of our invention would not be corroded by salt spray after a period of more than six hours in a salt water spray test (the concentration of salt in water, the temperature within the test tank, 35 C.; and spray pressure, 20 p.s.i.), and exhibits lacquer adherence as strong as those treated with prior phosphating methods, including strong resistance to ate acidity 3,032,487 Patented May 1, 1962 ice corrosion of the base metal under the lacquer or pain coatings. v

The present invention requires the combination of four indispensable factors: trivalent chromium ions, phosphoric acid, boric acid, and cathodic electrolysis. The absence of any one of the above-mentioned factors in the treatment process has resulted in an inferior protective coating than that of the instant invention.

An embodiment of the invention is hereinbelow.

The solution employed consists of 10-20 g./l. of chromic anhydride, 3-10 g./l. of phosphoric acid, and l-20 g./l. of boric acid. Further, this solution may be added with either a weak chromic acid of trivalent chromium or a reducing agent, such as, oxalic acid having an appropriso as to reduce a part of chromate and include 0.5-5.0 g./l., and preferably 2 to 3 g./l. of trivalent chromium ions, and this solution has a pH value of from 0.5 to 2.5. Instead, the trivalent chromium ions, again preferably in amounts of 2 to 3 g./l. can be introduced into the solution by adding thereto a corresponding amount, i.e. about 9.5 to 14.25 g./l. of chromic acetate [calculated as Cr(OOC.CH .H O].

Referring to the concentration of chromic anhydride, if it is higher than that specified above, paint adherence is deteriorated while, if lower, corrosion resistance is lowered. Further, if the concentration of phosphoric acid is higher than that specified above, shalky films are formed to reduce corrosion resistance While, if lower, corrosion resistance also decreases. Referring to the concentration of boric acid, on the other hand, it seems that the higher or lower concentration thereof than that specified above deteriorates corrosion resistance.

A ferrous metal article is cathodically electrolyzed in the above aqueous solution at a current density of from 10 to 300 amps. p.s.f. at a temperature of from 15 to C. for a period of 0.25 second or more. The treated fer rous metal article removed from the solution may be squeezed in order to remove any excess solution present thereon, and is dried. The resulting product on the surface of a ferrous metal article after drying has a unique surface film which is excellent in paint adherence as well as in corrosion resistance.

The exact nature of the reaction or the formation of the coating film is not known precisely, but it is believed that the cathodic electrolytic action to which the ferrous metal is subjected increases the value of pH around the surface of the metal so as to precipitate chromium phosphate and a gel-like chromium chromate due to the chemical reaction of trivalent chromium ion as Well as phosphate ion in the electrolytic solution, whereby the chromium phosphate adheres closely and tightly to the ferrous metal in order to convert into a crystalline thin film of phosphate over which, it is also believed, the sol-like chromium chromate covers. On drying the sol-like film of chromium chromate, it becomes a gel and produces a water-repellent film so as to improve its corrosion resistance as well as its paint adhesion.

Furthermore, the exact nature of the action of boric acid is not 'known precisely, but it is considered that it exerts a favorable benefit to the control of the pH value in the solution and also to the formation of the film by gelling through dehydration.

Our invention will be more readily understood by described in detail referring to the following examples. Full bright finish steel (blackplate) was employed in the examples.

Example 1 1 Chromic anhydri 15 Phosphoric acid 5 Boric aci 5 Oxalic acid 7.5-

The above four ingredients are mixed and boiled to prepare the electrolytic solution in which oxalic acid is completely oxidized by chromic acid.

Temperature room temperature. Current density 60 amps. p.s.f. Treating time 1 second or more.

Steel is cathodically electrolyzed according to the above.

Steel is cathodically treated according to the above.

We claim:

1. In a method of electrolytically treating the surface of a ferrous metal article, the improvement of (1) immersing said article in an aqueous solution having a pH of from about 0.5 to 2.5 and consisting essentially of 25 10 to 20 grams per liter of chromic anhydride, 1 to 5 grams per liter of trivalent chromium ions, 3 to grams per liter of phosphoric acid and 1 to 20 grams per liter of boric acid; and (II) subjecting the ferrous metal article to electrolysis as cathode in the said solution at a current density of from 10 to 300 amps. p.s.f. at a temperature of from to 80 C., for a time of more than 0.25 second.

2. The improvement described in claim 1, wherein the time of subjecting said ferrous metal article to electrolysis as cathode ranges from 0.25 to about 1 second.

3. The improvement described in claim 1, wherein said solution is prepared by dissolving in 1000 parts of water from about 10 to parts by weight of chromic anhydride, about 3 to 10 parts by weight of phosphoric acid about 1 to 20 parts by weight of boric acid and additionally oxalic acid in such amount as to convert from 2 to 3 grams per liter of chromium ions present in the solution from hexavalent to trivalent ions.

4. The improvement described in claim 1, wherein said solution is prepared by dissolving in water for every 1000 parts of volume of solution from about 10 to 20 parts by Weight of chromic anhydride, about 3 to 10 parts by weight of phosphoric acid, about 1 to 20 parts b weight of boric acid, and about 9.5 to 14.25 parts by weight of chromic acetate, thereby providing for the presence, in each liter of the resulting aqueous solution, of from about 2 to 3 grams of trivalent chromium ions.

5. In a method of electrolytically treating the surface of a ferrous metal article, the improvement of (I) immersing said article in an aqueous solution containing as the sole essential components dissolved therein per 1000 parts by volume thereof: 15 parts by weight of chromic acid anhydride, 5 parts by weight of phosphoric acid, 5 parts by weight of boric acid, and 7.5 parts by weight of oxalic acid, and (II) subjecting the said article to electrolysis as cathode in said solution at a current density of about 60 amps. p.s.f for about 1 second and at room temperature.

6. In a method of electrolytically treating the surface of a ferrous metal article, the improvement of (1) immersing said article in an aqueous solution containing as the sole essential components dissolved therein per 1000 parts by volume thereof: 12 parts by weight of chromic acid anhydride, 10 parts by weight of phosphoric acid, 5 parts by weight of boric acid, and 9.5 parts by weight of chromic acetate, and (II) subjecting the said article to electrolysis as cathode in said solution at a current density of about 60 amps. p.s.f. for about 1 second and at a temperature of about C.

References Cited in the file of this patent UNITED STATES PATENTS 2,733,199 Wick a Jan. 31, 1956 2,769,774 Loveland et al. Nov. 6, 1956 2,812,296 Neish Nov. 5, 1957 2,812,297 Stareck et al. Nov. 5, 1957. 

1. IN A METHOD OF ELECTORLYTICALLY TREATING THE SURFACE OF A FERROUS METAL ARTICLE, THE IMPROVEMENT OF (1) IMMERSING SAID ARTICLE IN AN AQUEOUS SOLUTION HAVING A PH OF FROM ABOUT 0.5 TO 2.5 AND CONSISTING ESSENTIALLY OF 10 TO 20 GRAMS PER LITER OF CHRMIC ANHYDRIDE, 1 TO 5 GRAMS PER LITER OF TRIVALENT CHROMIUM OINS, 3 TO 10 GRAMS PER LITER OF PHOSPHORIC ACID AND 1 TO 20 GRAMS PER LITER, OF BORIC ACID; AND (II) SUBJECTING THE FERROUS METAL ARTICLE TO ELECTROLYSIS AS CATHODE IN THE SAID SOLUTION AT A CURRENT DENSITY OF FROM 10 TO 300 AMPS. P.S.F. AT A TEMPERATURE OF FROM 15 TO 80*C., FOR A TIME OF MORE THAN 0.25 SECOND. 